const target = [1,2,3,4,5,7]
const handler = {
    get(target, index, receiver) {
        if (index < 0) {
            //? index是个string 得parseInt
            return target[target.length + parseInt(index)]
        }
        return target[index]
    }
}
const proxy = new Proxy(target, handler)
console.log(proxy[-2]);
